1. Technical Field of the Invention
The present invention relates to an optical disk recording apparatus and, more particularly, to an improvement of write control (write strategy) of a CD-RW drive.
2. Prior Art
In a CD-RW, a phase-transition material is used for a recording layer, and when heated and then rapidly cooled, the phase-transition material is phase-changed from a crystalline state to an amorphous state to form pits. In addition, when the phase-transition material is heated and gradually cooled, the pits phase-changed into the amorphous state are returned to the crystalline state to thereby accomplish erasing of pits.
The crystalline and amorphous states are different in reflectivity by about 15 percent, which thus enables record/erase of information in the form of pits.
The recording layer is heated by applying a laser light, but in this case, there exists a disadvantage that, during writing, the rapid cooling is not satisfactorily achieved when a high power (write power) laser light is continuously applied and the recording layer that has been made amorphous once is turned again to the crystalline state, resulting in failure to form the pits. Now, write control (write strategy) of the CD-RW is performed, as shown in FIG. 7(A), by a multi-pulse method in which a laser light of write power is intermittently applied in a pulsed manner and the power level is lowered to bottom power between pulses to help the recording layer be cooled down rapidly.
A conventional CD-RW drive employs such a 1T multi-pulse write strategy that one cycle of a pulse (pulse width+pulse interval) corresponds to one clock cycle (1T) of EFM.
Furthermore, in cooling the recording layer, the electric current of a semiconductor laser diode is not completely cut off but is made to flow at bottom power so that a rise to the next write power may be promptly performed. In addition, for overwrite, portions where the pits are not to be formed (portions which become lands) are irradiated with a laser light of erase power to erase the old pits.
On the other hand, with a writing speed of writing information into the CD-RW becoming higher, 10 times to 24 times writing speed is required. When writing is performed at 24 times speed, a temporal length for 1T is about 9.6 nS. In the meantime, times needed for the semiconductor laser to turn on (a rise from bottom power to write power) and to turn off (a fall from write power to bottom power) are both about 2 nS. Therefore, when the write strategy for 1T multi-pulse is performed at 24 times speed to raise a pulse of 0.5T width, the pulse finishes before laser power rises as shown in FIG. 7(B), which thus causes a problem that good pits cannot be formed.
One possible idea to cope with this problem is to make a pulse cycle longer, but when the pulse cycle is made longer than 1T, a bit length might not synchronize with the pulse cycle, which means, for example, that a 3T-pit cannot be formed on a 2T-cycle, thus leading to a problem that a simple multi-pulse train does not allow pits having a correct length to be formed. Further, another problem is that the write strategy of the multi-pulse on a long pulse cycle does not enable pits having a good shape to be formed at about 1 to 10 times low recording speed.